The Vector Probe in Heavy-Ion Reactions

1
The Vector Probe in Heavy-Ion Reactions
Ralf Rapp Cyclotron Inst. Physics Dept. Texas
AM University College Station, Texas Hot
Quarks Workshop Taos Valley, 21.07.04
2
Introduction I E.M. Probes in Strong Interactions

• g-ray spectroscopy of atomic nuclei collective
  phenomena
• DIS off the nucleon - parton model, PDFs
  (high Q2)
• - nonpert.
  structure of nucleon JLAB
• thermal emission - compact stars (?!)
• - heavy-ion
  collisions
• What is the electromagnetic spectrum of QCD
  matter?

3
Outline
1. Introduction 2. Four Pillars of
Electromagnetic Radiation 3. Dileptons 3.1
Low Mass - Axial-/Vector Correlator and Chiral
Symmetry - Medium
Effects and Excitation Function
- Lattice QCD 3.2 Intermediate Mass
QGP Radiation? 3.3 RHIC 4. Photons 4.1 QGP
and Hadron Gas Emission 4.2 SPS and RHIC
Phenomenology 5. Conclusions
4
Introduction II Electromagnetic Emission Rates
E.M. Correlation Function
Im ?em(M,q)
Im ?em(q0q)
also e.m susceptibility (charge fluct) ?
?em(q00,q?0)

• In URHICs
• source strength dependence on T, mB, mp
  medium effects,
• system evolution V(t), T(t), mB(t)
  transverse expansion,
• nonthermal sources ee- Drell-Yan,
  open-charm(!) g initial/
• consistency!
  pre-equil.

5
2.) Four Pillars of Thermal E.M. Radiation
Thermal rate
q00.5GeV ? Tmax0.17GeV , q01.5GeV ?
Tmax0.5GeV
6
3.) Low-Mass Dileptons Chiral Symmetry
Im ?em(M) Im Dvec(M) vector-meson spectral
functions dominated by r-meson ? chiral
partner a1(1260)
Vacuum
pQCD cont.
Chiral breaking Q2 lt 3GeV2
7
3.1.1 Vector Mesons in Medium Many-Body Theory
Constraints - branching ratios B,M?rN,rp - gN,
gA absorpt., pN?rN - QCD sum rules, lattice
B,a1,K1...

N,p,K
8
(ii) Vector Mesons at RHIC
9
3.1.2 Low-Mass Dileptons in URHICs
Top SPS Energy

• baryon effects important!

10
3.1.3 Current Status of a1(1260)
11
3.1.4 Comparison of Hadronic Models to LGT
12
3.2 Intermediate-Mass Dileptons NA50 (SPS)
e.m. corr. continuum-like Im ?em M2 (1as/p)
QGP HG!
13
3.3 Dilepton Spectrum at RHIC
14
4.) Thermal Photons
Quark-Gluon Plasma
Naïve LO q q (g) ? g (q) ?
But other contributions in O(as) collinear
enhanced Dg(t-mD2)-11/as
Bremsstrahlung Pair-ann.scatt.
ladder resummation (LPM)
Aurenche etal 00, Arnold,MooreYaffe 01
15
4.2 Comparison to Data I WA98 at SPS
Hydrodynamics QGP HG
Huovinen,RuuskanenRäsänen 02

• T0260MeV, QGP-dominated
• still true if pp?gX included

16
4.2 Comp. to Data II WA98 Low-qt Anomaly
Expanding Fireball Model
Turbide,RRGale04

• current HG rate much below
• 30 longer tFB ? 30 increase

17
4.2 Perspectives on Data III RHIC
Predictions for Central Au-Au
PHENIX Data

• large pre-equilibrium yield
• from parton cascade (no LPM)
• thermal yields consistent
• QGP undersat. small effect

• consistent with pQCD only
• disfavors parton cascade
• not sensitive to thermal yet

18
5.) Conclusions

• Thermal E.M. Radiation from QCD matter
• - hard high-E photons, intermediate-M
  dileptons pQCD
• QGP radiation?!
• - soft low-E g , low-M ll- Pem(M,q)
  
• chiral restoration?!

• extrapolations into phase transition region
• ? in-med HG and QGP shine equally bright
• lattice calculations? deeper reason?

• phenomenology for URHICs promising
• precision datatheory needed for definite
  conclusions
• much excitement ahead PHENIX, NA60, HADES,
  ALICE,
• and
  theory!

19
Additional Slides
20
2. Thermal Photon Radiation
2.1 Generalities
Emission Rate per 4-volume and 3-momentum
transverse photon selfenergy
many-body language
in-medium effects, resummations,
21
2.3.1 Hot Hadronic Matter p-r-a1 Gas
Chiral Lagrangian Axial/Vector-mesons, e.g. HLS
or MYM

• (g0,m0,s,x) fit to mr,a1 , Gr,a1
• D/S and G(a1?p?) not optimal

Song 93, Halasz etal 98,

• Photon-producing reactions

mostly at dominant (q0gt0.5GeV)
gauge invariance! q0lt0.5GeV a1-strength
problematic
22
2.3.1.b Hadronic Formfactors

• quantitative analysis account for finite
  hadron size
• improves a1 phenomenology
• t-channel exchange gauge invariance nontrivial
  Kapusta etal 91
• simplified approach
  Turbide,GaleRR 04

with
23
2.3.2 Further Meson Gas Sources
(i) Strangeness Contributions SU(3)F MYM
25 of pp???
40 of pr?p? !
(iii) Higher Resonances Ax-Vec a1,h1?pg,
Vec w,w,w?pg other p(1300)?pg
f1?rg , K1?Kg K?Kg
a2(1320)?pg
24
2.3.3 Baryonic Contributions

• use in-medium r spectral funct
• constrained by nucl. g-absorption

B,a1,K1...
N,p,K
25
2.3.3(b) Photon Rates from r Spectral
FunctionBaryons Meson-Resonances

• baryonic contributions
• dominant for q0lt1GeV
• (CERES enhancement!)
• also true at RHICLHC
• at T180MeV, mB0

mB220MeV
26
2.3.4 HG Emission Rates Summary

• w t-channel (very) important
• at high energy
• formfactor suppression (2-4)
• strangeness significant
• baryons at low energy

mB220MeV
Turbide,RRGale 04
27
2.3.5 In-Medium Effects

• many-body approach encoded in vector-spectral
  function,
• 
  relevant below M , q0 1-1.5 GeV
• dropping masses
• large enhancement due
• to increased phase space
• SongFai 98, Alam etal 03
• unless
• vector coupling decreases
• towards Tc (HLS, a?1)
• HaradaYamawaki 01,
• Halasz etal 98

28
2.3.6 Hadron Gas vs. QGP Rate

• complete in-med QGP rate
• factor 2 larger than
• naïve LO QGP
• total HG rate (bottom-up)
• very similar to
• in-med QGP (top-down)
• quark-hadron duality ?

29
3.2 Thermal Evolution QGP? Mix? HG
QGP initial conditions SPS

• t01fm/c ? t00.5fm/c 2-3
• sCdQGT3 dQG40 ? 32 2
• pre-equilibrium?!

30
4.2 Non-Thermal Photon Sources
Initial hard production pp ? ?X
scaling with xT2pT /vs , power-law fit
Srivastava 01
31
Photon Properties in Colorsuperconductors
32
2.2.4 In-Medium Baryons D(1232)

• ? long history in nuclear physics ! ( pA , gA
  )
• e.g. nuclear photoabsorption MD, GD up by
  20MeV
• ? little attention at finite temperature
• ? D-Propagator at finite rB and T van
  Hees RR 04

33
(ii) D(1232) in URHICs
? broadening Bose factor, pD?B ? repulsion
pDN-1, pNN-1
not yet included
(pN?D)
34
3.1 Continuity?!
E.M. Emission Rates
Light Hadron Masses
Shuryak, Zahed, Brown 04
35
2.2.6 Observables in URHICs
e e-
?

• (i) Lepton Pairs
  (ii) Photons

Turbide,GaleRR 03

• consistent with dileptons
• pp Brems with soft s at low q?

baryon density effects!
36
3.3 Dilepton Spectrum at RHIC
37
4.3 Perspectives on Data III RHIC
Predictions for Central Au-Au
PHENIX Data

• large pre-equilibrium yield
• from parton cascade (no LPM)
• thermal yields consistent
• QGP undersat. small effect

• consistent with initial only
• disfavors parton cascade
• not sensitive to thermal yet

38
(No Transcript)